Metallomics, the comprehensive analysis of all metal species within a cell or tissue, and metalloproteomics, the study of the part of metallome that involves the protein ligands, heteroatom-tagged proteomics are emerging areas in speciation analysis in bioinorganic chemistry for clinical research and pharmaceutical research. In such analytical studies, both elemental (e.g., metallic) and molecular information of an analyte needs to be investigated.
Currently, the analytical techniques for this type of research typically use several different methods and instruments, either separately or in certain combination. For instance, inductively coupled plasma mass spectrometry (ICP-MS) is used for metal detection and the electrospray ionization (ESI) or matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) is used for identification of the biomolecules.
The ICP has been a powerful tool for analytical chemistry since it was introduced as an atomic emission source for optical spectrometry in the 1960's. In the ICP, radio frequency electrical energy is continuously coupled via a spiral load coil into an inert gas flow stream at atmospheric pressure. In a typical design, argon flows through a plasma torch made of three concentric quartz tubes (e.g., U.S. Pat. No. 3,958,883) and placed within a spiral coil. The central gas flow usually referred to as the carrier gas flow because it is used to carry the sample to be ionized or excited. An intermediate gas flow, termed the auxiliary flow, is needed for confining the hot carrier gas and cooling. The outermost flow, termed the coolant gas, both sustains the plasma and protects the quartz tube from melting due to the high temperature in the plasma.
The electromagnetic field induced by the radio frequency energy (e.g., 27 MHZ or 40 MHZ) sustains a plasma in the gas. The plasma contains free electrons, ions, and excited atoms and molecules. A chemical sample, usually in a form of aerosol droplets, is introduced into the carrier gas stream through the central quartz tube into the plasma. The aerosol sample is vaporized and decomposed to atoms and small molecules, due to the high temperature of the plasma. Some of the atoms and molecules of the sample are further excited and ionized by the free electrons.
ICP-MS is widely accepted for its isotope specificity, multi-element capability, high sensitivity, wide dynamic range (i.e., about 9 orders of magnitude), and the fact that the signal intensity is independent of the molecular structure of the protein and sample matrix.
ESI-MS has proven to be very valuable in system biology and genomic studies since provides information on the protein identification. However, ESI-MS is strongly compound dependent and often suppressed by the presence of the matrix. When the information generated by the two techniques is combined it has been shown to help with the identification of metalloproteins and their functions at the molecular level.
Because of the complexity of the human serum, a multidimensional separation, such as high performance liquid chromatography (HPLC) is usually employed prior to mass spectrometric detection. In conventional methods, the effluent of a chromatographic column is then either split between the ICP-MS and the ESI-MS systems, or fractions are collected and chromatographed again under a different set of operating conditions prior to electrospray MS. In both cases, at least two mass spectrometers are needed, which increases costs and maintenance. The setup is complicated and sample analysis is very time consuming.
Therefore, there is a need in the art to address the aforementioned deficiencies and shortcomings.